1. Field of the Invention
The present invention relates to a superconducting wire, and more particularly, it relates to a superconducting wire which comprises a base material and an oxide superconducting layer formed on the base material.
2. Description of the Background Art
Metal, compound and ceramic materials have been generally known as superconductors, and study has been made to apply such superconductors to various uses. A superconductor loses all resistance to electric current when the same is maintained at a temperature lower than its critical temperature, and attempts have been made for applying such property to generation of a high magnetic field, high-density bulk power transmission and the like.
A ceramic superconductive material has been recently highlighted in such a point that its critical temperature for a superconducting phenomenon can be increased. Such a superconductive material can be worked into an elongated linear member, for example, for application to power transmission/distribution, electrical connection of various types of devices or elements, line winding or the like.
In order to work a conventional compound superconductive material into such a wire rod, there has been proposed a method of forming a layer of the superconductive material on a base material of stainless steel etc. or an alloy tape by sputtering or the like. Excellent flexibility can be easily attained by forming the superconducting layer on such a tape-type base.
If a layer of a ceramic superconductive material is formed on a general metal or ceramic tape, however, it is impossible to attain good superconductivity particularly when the superconducting layer has a small thickness of not more than 1 .mu.m.
For example, Japanese Patent Laying-Open Gazette No. 271816/1988 discloses a superconducting wire which comprises an oxide single crystal fiber member and a layer of an oxide superconductor formed on the oxide single crystal fiber member. Such an oxide superconducting layer can be formed by a vapor-phase thin film forming method such as vapor deposition, sputtering, CVD or the like.
In order to attain high critical current density in such a superconducting wire, however, it is generally necessary to perform heat treatment at a high temperature. Also when a layer of an oxide superconductor is formed on a single crystal member of MgO, SrTiO.sub.3 or the like, which can provide a film of excellent crystallinity, it is necessary to keep a substrate temperature of 600.degree. to 700.degree. C. during film formation and to further perform heat treatment at a temperature of at least 900.degree. C. after film formation.
When a layer of an oxide superconductor is formed on a substrate of a flexible metal, ceramic material or glass, heat treatment must be performed at a higher temperature for a longer time to compensate for inferior crystallinity. If such high-temperature heat treatment is performed for a long time, however, diffusion reaction is caused between the superconducting layer and the substrate, to deteriorate superconductivity of the oxide superconductor layer. In order to prevent such diffusion reaction, an intermediate layer may be formed between the substrate and the oxide superconductor layer. However, such an intermediate layer must be at least 1 .mu.m in thickness to withstand the high-temperature heat treatment which is performed for a long time, and hence the cost for forming the intermediate layer is increased.